The spiral wound fuses disclosed in the above-identified patents have a cylindrical, transparent main body enclosed by cup-shaped terminal-forming metal end caps between which is soldered a fuse wire assembly extending tautly between the terminals. The fuse wire assembly includes a core made from a limp twisted bundle of ceramic yarn devoid of any sizing or the like. Fuse wire (sometimes referred to as a fuse filament) is spirally wound upon this limp bundle of twisted ceramic yarn to form a semi-rigid body which can maintain its position when soldered between the end caps described. The purpose of the insulating core is to act as a heat sink so that the fuse has slow blow characteristics under modest overload conditions.
Commonly, in slow blow fuses of the type just described the fuse wire comprises a tin plated copper wire. (Typically, the tin plating increases the thickness of the bare copper wire by a factor of about 1.16.) The tin plating material when it migrates into and alloys with the copper of the fuse wire, serves the function of increasing the resistance and reducing the melting temperature of the coated copper wire from that of the copper without the tin plating thereon. The tin plating material desirably remains as a coating on the base copper metal of the fuse wire until the coated wire is heated to a given high temperature by a given percent overload current flowing for a given minimum period of time. The tin then migrates at appreciable rates into the copper metal wire to form the copper-tin alloy which has a melting temperature much lower than the melting temperature of the pure copper. Thus, if this overload current persists for this period of time, the melting temperature of the copper alloy is reached and the fuse blows.
The migration rate of the tin plating can vary along different points of the tin plated copper wire, dependent upon the temperature at those points. Also, if there are imperfections like indentations at points in the copper wire, it will take a lesser time at a given temperature and amount of tin for the tin to migrate completely into the wire and produce a blown fuse wire. Such imperfections thus can undesirably cause a fuse to blow prematurely.
The most serious problem in slow blow fuses having tinned or similar coatings which alloy with the base metal to lower its melting temperature is that the migration of these coating materials is an irreversible process, and that it occurs, though more slowly under current flow conditions even below overload current, that is at rated current and below. Therefore, all slow blow fuses degrade with time. Thus, to maximize fuse life and to increase the margin of safety, fuse manufacturers commonly recommend that a fuse having a given rating be placed in circuits where normal current flow does not exceed 0.8 of its rating. The migration of the tin or other similar coatings occurs even at these lower current levels, so that the fuse still progressively degrades with use, whereby it undesirably blows at normal current levels if it is used long enough.
Still another problem which sometimes occurs due to the tin plating is that an undesirably thick coating of the tin plating can cause the tin plating to ball-up between turns of the spiral wound fuse wire and thereby short circuit the fuse wire before the blowing temperature is reached. In such case, the blowing conditions become modified which makes the fuse involved unreliable to perform its intended function.
It has been discovered that all of the above problems become exacerbated when two tin plated fuse wires are placed in intimate contact with one another, so that there are two thicknesses of tin plate between the copper wires. This occurs in a shunt fuse where the aforesaid tinned spiral fuse is wound over another similar tinned fuse wire to increase its current handling capacity.
In one commercial shunt fuse, the latter wire was a single straight tinned fuse wire extending axially along the insulating core of the fuse. Thus, each turn of the tinned spiral wound fuse wire crosses and contacts the straight fuse wire at spaced points therealong, to form a series of parallel connected fuse wire segments extending along the length of the fuse and separated by two layers of tin.